Dry-transfer of chemical vapour deposited nanocarbon thin films

Abstract

This thesis presents the development of chemical vapour deposited (CVD) graphene and multi-walled carbon nanotubes (MWCNTs) as enabling technologies for flexible transparent conductors offering enhanced functionality. The technologies developed could be employed as thin film field emission sources, optical sensors and substrate-free wideband optical polarisers. Detailed studies were performed on CVD Fe and Ni catalysed carbon nanotubes and nanofibres on indium tin oxide, aluminium and alumina diffusion barriers. Activations energies of 0.5 and 1.5 eV were extracted supporting surface diffusion limited catalysis for CNTs and CNFs. For the first time an activation energy of 2.4 eV has been determined for Cu-catalysed growth of CVD graphene. Graphene was shown to deviate significantly from the more traditional rate-limited surface diffusion and suggests carbon-atom-latticeintegration limited catalysis. An aligned dry-transferred MWCNT thin film fabrication technique was developed using MWCNTs of varied lengths to control the optical transparency and conductivity. A process based on the hot-press lamination of bilayer CVD graphene (HPLG) was also developed. Transport studies revealed that these thin films behave, in a macroscopic sense, similar to traditional c-axis conductive graphite and deviate toward tunnel dominated conduction with increasing degrees of network disorder. Various MWCNT-based thin film field emitters were considered. Solution processing was shown to augment the surface work function of the MWCNTs resulting in reduced turn-on electric fields. Integrated zinc oxide nanowires were investigated and were shown to ballast the emission, thereby preventing tip burn out, and offered lower than expected turn-on fields due to the excitation of a hot electron population. To obviate nearest neighbour electrostatic shielding effects an electrochemical catalyst activation procedure was developed to directly deposit highly aligned sparse carbon nanofibres on stainless steel mesh. Highly-aligned free-standing MWCNT membranes were fabricated through a solid-state peeling technique. Membranes were spanned across large distances thereby offering an ideal platform to investigate the unambiguous photoresponse of MWCNTs by removing all extraneous substrate interfaces, charge traps and nanotube-electrode Shottky barriers as well as using pure, chemically untreated material. Oxygen physisorbtion was repeatedly implicated through in-situ lasing and in-situ heated EDX measurements, FT-IR and lowtemperature transport and transfer measurements. A MWCNT membrane absorptive polariser was fabricated. Polarisers showed wideband operation from 400 nm to 1.1 μm and offered operation over greater spectral windows than commercially available polymer and glass-support dichroic films. Ab-initio simulations showed excellent agreement with the measured polarisation attributing the effect to long-axis selective absorption.